Gaseous waste products resulting from the combustion of hydrocarbonaceous fuels, such as gasoline and fuel oils, comprise carbon monoxide, hydrocarbons and nitrogen oxides as products of combustion or incomplete combustion, and pose a serious health problem with respect to pollution of the atmosphere. While exhaust gases from other carbonaceous fuel-burning sources, such as stationary engines, industrial furnaces, etc., contribute substantially to air pollution, the exhaust gases from automotive engines are a principal source of pollution. Because of these health problem concerns, the Environmental Protection Agency (EPA) has promulgated strict controls on the amounts of carbon monoxide, hydrocarbons and nitrogen oxides which automobiles can emit. The implementation of these controls has resulted in the use of catalytic converters to reduce the amount of pollutants emitted from automobiles.
In order to achieve the simultaneous conversion of carbon monoxide, hydrocarbon and nitrogen oxide pollutants, it has become the practice to employ catalysts (known as three component control catalysts) in conjunction with air-to-fuel ratio control means which function in response to a feedback signal from an oxygen sensor in the engine exhaust system. Although these three component control catalysts work quite well after they have reached operating temperature of about 300.degree. C., at lower temperatures they are not able to convert substantial amounts of the pollutants. What this means is that when an engine and in particular an automobile engine is started up, (cold start) the three component control catalyst is not able to convert the hydrocarbons and other pollutants to innocuous compounds. Despite this limitation, current state of the art catalysts are able to meet the current emission standards. However, California has recently set new hydrocarbon standards (which most probably will be promulgated nationwide) which can not be met with the current state of the art three component control catalysts.
The prior art reveals several references which disclose the use of an adsorbent bed to minimize hydrocarbon emissions during a cold start engine operation. One such reference is U.S. Pat. No. 3,699,683 in which an adsorbent bed is placed after both a reducing catalyst and an oxidizing catalyst. The patentees disclose that when the exhaust gas stream is below 200.degree. C. the gas stream is flowed through the reducing catalyst then through the oxidizing catalyst and finally through the adsorbent bed, thereby adsorbing hydrocarbons on the adsorbent bed. When the temperature goes above 200.degree. C. the gas stream which is discharged from the oxidation catalyst is divided into a major and minor portion, the major portion being discharged directly into the atmosphere and the minor portion passing through the adsorbent bed whereby unburned hydrocarbons are desorbed and then flowing the resulting minor portion of this exhaust stream containing the desorbed unburned hydrocarbons into the engine where they are burned.
Another reference is U.S. Pat. No. 2,942,932 which teaches a process for oxidizing carbon monoxide and hydrocarbons which are contained in exhaust gas streams. The process disclosed in the patent consists of flowing an exhaust stream which is below 800.degree. F. into an adsorption zone which adsorbs the carbon monoxide and hydrocarbons and then passing the resultant stream from this adsorption zone into an oxidation zone. When the temperature of the exhaust gas stream reaches about 800.degree. F. the exhaust stream is no longer passed through the adsorption zone but is passed directly to the oxidation zone with the addition of excess air.
Canadian Patent No. 1,205,980 discloses a method of reducing exhaust emissions from an alcohol fueled automotive vehicle. This method consists of directing the cool engine start-up exhaust gas through a bed of zeolite particles and then over an oxidation catalyst and then the gas is discharged to the atmosphere. As the exhaust gas stream warms up it is continuously passed over the adsorption bed and then over the oxidation bed.
Finally, U.S. Pat. No. 4,985,210 discloses the purification of automotive exhaust by flowing the exhaust through an adsorbent bed and then through a catalyst. The adsorbent bed contains a mordenite or a Y-type zeolite.
Applicant has solved the cold start problem in a way that differs significantly from the prior art. Applicant's invention involves directing an engine exhaust gas stream during cold start operation over a catalyst; taking the gas stream discharged from the catalyst (first exhaust stream) and then flowing it over the turbine side of a turbocharger. After flowing through the turbine, the exhaust stream (second exhaust stream) is flowed over an adsorbent bed and then discharged to the atmosphere. The adsorbent bed preferentially adsorbs hydrocarbons instead of water at the conditions present in the exhaust stream. The adsorbents which may be used to adsorb the hydrocarbons may be selected from the group consisting of molecular sieves which have 1) a Si:Al ratio of at least 2.4; 2) are hydrothermally stable; and 3) have a hydrocarbon selectivity greater then 1. Examples of molecular sieves which meet these criteria are silicalite, faujasites, clinoptilolites, mordenites and chabazite. The adsorbent bed may be in any configuration with a preferred configuration being a honeycomb monolithic carrier having deposited thereon the desired molecular sieve. After a certain amount of time, the adsorbent bed has reached a temperature (about 150.degree. C. to 200.degree. C.) at which the bed is no longer able to remove hydrocarbons from the engine exhaust stream. That is, hydrocarbons are actually desorbed from the adsorbent bed instead of being adsorbed. At that point the second exhaust stream discharged from the turbine is divided into a major and minor component. The major component is discharged to the atmosphere, while the minor component is flowed through the adsorbent bed. The exhaust stream from the adsorbent bed (third stream) is now flowed through the compressor side of the turbocharger and then directed back over the catalyst. When all the hydrocarbons have been desorbed from the adsorbent bed, the second exhaust stream is completely diverted around the adsorbent bed and discharged to the atmosphere.
It is apparent that applicant's invention differs significantly from the prior art. One difference is that the adsorbent used in the instant process is one that must meet the three criteria stated above. Another difference is that the instant invention uses a turbocharger to recycle the exhaust stream from the adsorbent bed, which contains desorbed hydrocarbons, through the catalyst so that these hydrocarbons can be converted to innocuous compounds and not contribute to the emissions which are discharged to the atmosphere.